Ferroelectric interfaces

From the data listed in Table 3, it may be noted that the polarization values of the ceramics are lower than that of the single crystal [7,8] whereas the coercive field is higher. This may be attributed to the existence of the non-ferroelectric layers at the metal-ferroelectric interfaces and grain boundaries. In other way, when the anisotropy of the crystal get stronger, the displacement of ions, which is demanded by polarization inversion, get larger, the coercive field will be stronger. 

The value of of the BaTiO, ceramics is lower than that reported for BaTiOj single crystal [9] (along [100] =4 000). This may be due to the structural and compositional variances. Meanwhile, the size of crystalline particle may affect the dielectric constant , that is, when the particle size is lower than certain value, the constant will decrease with the decrease of the size. In addition to those mentioned above, porosity and the existence of low dielectric constant affect the non-ferroelectric layers at the metal-ferroelectric interface and the grain boundaries. 

Two competing types of phenomena that arise at the ferroelectric interfaces can affect the properties of the superlattices. The strain field, generated by the mechanical mismatch between superlattice layers, influences the polarization orientation and generally increases the ferroelectric transition temperature Tc [32]. In contrast, the electric depolarization field, produced by interfacial surface charges is unfavorable to formation of the ferroelectric phase [33]. In fact, in cubic perovskite-like ferroelectrics the situation can be even more complex due to both 180° ferroelectric [27] and 90° ferroelastic [21, 32, 34] domain formation. 

The term photovoltaic effect is further used to denote non-electrochemical photoprocesses in solid-state metal/semiconductor interfaces (Schottky barrier contacts) and semiconductor/semiconductor pin) junctions. Analogously, the term photogalvanic effect is used more generally to denote any photoexcitation of the d.c. current in a material (e.g. in solid ferroelectrics). Although confusion is not usual, electrochemical reactions initiated by light absorption in electrolyte solutions should be termed electrochemical photogalvanic effect , and reactions at photoexcited semiconductor electodes electrochemical photovoltaic effect . 

It is interesting to point out here that with all of the theoretical speculation in the literature about polar order (both ferroelectric and antiferroelectric) in bilayer chevron smectics, and about reflection symmetry breaking by formation of a helical structure in a smectic with anticlinic layer interfaces, the first actual LC structure proven to exhibit spontaneous reflection symmetry breaking, the SmCP structure, was never, to our knowledge, suggested prior to its discovery. 

Apparently this switching mode is disfavored since, in fact, the chirality of the layers does not change upon switching to the ferroelectric state rather the layer interface clinicity changes. This occurs when the molecules in alternate layers simply precess about the tilt cone in a manner exactly analogous to antiferroelectric to ferroelectric switching in the chiral SmC phase. As shown in Figure 8.25, the ferroelectric state obtained from the ShiCsPa antiferroelectric phase is a ShiCaPf structure, an achiral macroscopic racemate with anticlinic layer interfaces. 

As can be easily seen by inspection of these illustrations of the SuiCsPa and ShiCsPf phases, while the director tilt in the tilt plane is synclinic for both, the layer interfaces have a different character when observed in projection in the bow plane. The antiferroelectric diastereomer has synclinic character at the layer interfaces, while the ferroelectric diastereomer is anticlinic in the bow plane. This suggests a very simple reason for the tendency toward antiferroelectric bananas, this being basically the same as the tendency toward ferroelectric calamitic smectics preference for synclinic layer interfaces. 

From this discussion the clear similarity between the SmAPA and SmCsPA structures is easily seen. In addition, the suggestion of Brand et al.29 that a bilayer smectic with all anticlinic layer interfaces (the SmAPF) would produce an achiral ferroelectric smectic follows directly. The unanticipated tilt of the director in the tilt plane, leading to a chiral layer structure, seems to be a general response of the bent-core mesogens to the spontaneous nonpolar symmetry breaking occurring in these rigid dimer structures. 

Figure 8.29 Illustration showing how all-anticlinic bilayer smectic should be ferroelectric is given. In case of covalent dimers (bent-core mesogens), equilibrium tilt of director combined with anti clinic layer interfaces in bow plane provides SmCsPF ferroelectric banana structure.

Ferroelectric thin films considerably gain in interest within the last couple of years due to their potential application in nonvolatile random-accessmemory devices (FeRAM). Among potential candidates, PbZr. n i, (>> (pzt) is one of the most promising materials because of its large remanent polarization and low coercive field. However, pzt is also well known for its poor fatigue behavior on metal electrodes [1,2] and occurrence of size effects [3-5] which are well due to the ferroelectric/electrode interface properties [1-5]. 

Addressing the inspection to internal interfaces, however, as manifests the ferroelectric film/metal electrode interface, has so far mostly been restricted to investigations using transmission electron microscopy (tem) [6,7], Still the influence of preparation conditions on the nanoscopic properties of as-prepared cross sections for tem may be debated, in conjunction of how the electron beam does alter the local electronic and physical constitutions. 

In conclusion, we reported the investigation of inner and outer interfaces in pzt in order to quantity both the amount of effective ferroelectric polarization and change in dielectric properties. With pfm and kpfm we find a transition layer occurring at the Pt/PZT interface within... 

Frey, M.H. et al. (1998) The role of interfaces on an apparent grain size effect on the dielectric properties for ferroelectric barium titanate ceramics, Ferroelectrics, 206-207, 337-53. 

The technological and commercial interest in metal oxide surfaces and interfaces has also continued to grow. For decades, oxide surfaces have played a key role in corrosion protection, catalysis, sensors, fuel cells, ceramics, etc. Over the last few years, totally new devices and technologies that rely on the properties of oxide surfaces and interfaces have emerged. Non-volatile ferroelectric field-... 

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